According to research on dental diseases by the Ministry of Health and Welfare in Japan in 1993, the average life of a second molar is 45.8 years, that is, the second molar is lost 10.5 years earlier than other teeth which have an average life of 56.3 years. It is now a matter of common knowledge that the main reasons for losing teeth are dental caries and periodontal disease. It is said that dental caries and periodontal disease are caused mainly by dental plaque that forms on the surfaces of teeth. Daily brushing plays a major role in preventing the accumulation of plaque and the occurrence of dental diseases caused by such plaque. Periodic care by dental professionals also plays a significant role in preventing dental disease. One of the main methods for controlling plaque is brushing with a toothbrush, and a variety of different types of toothbrushes have been developed for this purpose.
It has been proposed heretofore to reduce the size of the brush head of the toothbrush in order to more easily reach the second molars. When the body of the brush head is made smaller, the number of bristles must be correspondingly reduced compared to the number of bristles used in a conventional full-sized brush head. During brushing, the reduced number of bristles bears the same amount of pressure as that applied to the larger number of bristles found in conventional toothbrushes. This tends to cause increased bristle bending and shortens the useful life of the toothbrush.
Japanese Laid-open Patent Application Hei 5-168528(168528/93) discloses a toothbrush which has two sections of bristles, the bristles of one section being longer than the bristles of the other section. While such a toothbrush may be more efficient in removing dental plaque from molars, it is less efficient in removing dental plaque from the other teeth. The presence of unremoved dental plaque from the other teeth can lead to dental diseases such as caries and periodontal disease.
On the other hand, it is easy to brush the flat surfaces of the teeth with conventional toothbrushes having uniform bristle length, but, since the sizes of the interdental spaces vary according to their positions in the mouth, such brushes do not effectively clean the interdental spaces between the teeth.